The present invention relates to polishing pads used for chemical-mechanical polishing of substrates. More particularly, the present invention relates to modified contact areas on a polishing pad surface to produce a more uniformly polished substrate surface.
Chemical mechanical polishing (sometimes referred to as "CMP") typically involves mounting a substrate faced down on a holder and rotating the substrate face against a polishing pad mounted on a platen, which in turn is rotating or is in orbital state. A slurry containing a chemical component that chemically interacts with the facing substrate layer and an abrasive component that physically removes that layer is flowed between the substrate and the polishing pad or on the pad near the substrate. In semiconductor wafer fabrication, this technique is commonly applied to planarize various wafer layers such as dielectric layers, metallization layers, etc.
FIG. 1 shows a wafer 12 undergoing CMP on a surface of a rotating polishing pad 10 used in a conventional CMP system, such as an Avanti 472, commercially available from Integrated Processing Equipment Corporation (IPEC) of Phoenix, Ariz.. In such conventional CMP systems, polishing pad 10 typically rotates during CMP about an axis that is perpendicular to and passes through a center point of the polishing pad surface and although it is not necessary, wafer 12 may rotate in the same direction. A rotating wafer 12 carves out on polishing pad 10 a wafer track area, which is defined by an inner boundary 16 and an outer boundary 14. Those skilled in the art will recognize that the width of the wafer track area might be larger than the diameter of the wafer because during CMP, the rotating wafer also oscillates from side to side in a radial direction of the polishing pad. FIG. 1 shows a wafer 12 in its displaced, oscillating position 12'. Thus, in the conventional CMP systems, the wafer is polished on the wafer track area of the polishing pad. FIG. 2A shows a front view of a polishing pad 20, e.g., 1C 1000 available from Rodel of Newark, Del., that is employed in modem CMP systems, such as the AvantGaard 676 also available from Integrated Processing Equipment Corporation (IPEC). A surface of polishing pad 20 includes a plurality of macrogrooves 22, microgrooves 24 and slurry injection holes 26. Macrogrooves 22 are shown in an X-Y configuration, i.e. vertical and horizontal macrogrooves intersect at various points to form a "grid", microgrooves 24 are oriented substantially diagonally relative to macrogrooves 22 and slurry injection holes 26 are positioned at various intersections of the vertical and horizontal macrogrooves 22. Those skilled in the art will recognize that the macrogrooves formed on the polishing pad surface are not limited to any particular configuration and may be obtained by a polishing pad manufacturer in other configurations, such as a spiral configuration.
FIG. 2B shows a cross-sectional view of a macrogroove 22 of FIG. 2A, which macrogroove is shaped like a square channel with sharp comers having a width (labeled "w") and a depth (labeled "d"). Macrogrooves 22 of FIG. 2A have a substantially uniform width and depth (of about 1 mm) throughout the substrate surface. The term "macrogroove spacing," as used herein refers to a space on the polishing pad surface separating two parallel and adjacent macrogrooves. For macrogrooves in an X-Y configurations as shown in FIG. 2A, macrogrooves spacings 22 are typically between about 5 and about 6 mm and substantially uniform throughout the polishing pad surface.
During a typical CMP process, polishing pad 20 does not rotate, but orbits around an axis that is perpendicular to the polishing pad surface. FIG. 2C shows a polishing pad 20 (macrogrooves 22, microgrooves 24 and slurry injection holes 26 are not shown to simplify illustration) of FIG. 2A in its orbital state and for exemplary purposes, reference number 20' denotes one position of polishing pad 20 as it orbits around an axis that is perpendicular to the polishing pad surface. In other words, during the orbital motion of the polishing pad, a center point 28 of polishing pad 20 moves in a circular path, as shown in FIG. 2C. Wafer 12 subjected to CMP on orbiting polishing pad 20 is positioned off-center, i.e. the center-point of wafer 12 does not coincide with the center point of polishing pad 20, but is near to the center-point of polishing pad 20. In the modern CMP systems, therefore, a wafer surface mostly contacts the center area of the polishing pad during CMP.
After polishing a significant number of wafers on the same polishing pad, e.g., the polishing pad of FIGS. 1 or 2A, the part of the polishing pad that contacts a center region of the wafer deteriorates to a greater extent than other regions of the polishing pad. By way of example, in FIG. 1, a center region of the wafer track deteriorates to a greater extent than other areas of the polishing pad. As a further example, a center region of the polishing pad of FIG. 2A deteriorates similarly to a greater extent. This deterioration is attributed primarily to a constant down force applied by the wafer during CMP.
Unfortunately, well before the end of a production lot draws near, the degraded polishing pad surface causes the wafers subjected to CMP to experience a slower film removal rate at the center region of the wafer relative to the edge or peripheral regions of the wafer surface, which phenomenon is known in the art as "center slow polishing." "Production lot" refers to a collection of wafers that are fabricated as a group under substantially similar conditions and may ultimately be sold. Center slow polishing is undesirable because it leads to a non-uniformly polished wafer surface, i.e. the center region of the wafer surface is not polished to the same extent as the peripheral region of the wafer. This prematurely ends the life of the polishing pad. In a typical wafer fabrication facility, where several CMP apparatus are employed, the replacement cost of polishing pads can be significant.
What is therefore needed is an improved polishing pad design for producing a uniformly polished substrate surface.